Resin pipes are commonly connected by using conventional methods such as those shown in FIGS. 12 and 13. In the method shown in FIG. 12, there are two resin pipes 1a, 1b which comprise resin pipe line 1. Resin pipe 1b is softened at end portion 1c by using a torch lamp or an electric resistance heater. End portion 1c of resin pipe 1b expands so that the inside diameter becomes slightly larger than the shape of resin pipe 1a. Then adhesive 20 is put on the inner surface of end portion 1c. The outside surface of resin pipe 1a is fit to the inside surface of resin pipe 1b. By hardening adhesive 20, the two resin pipes 1a, 1b are connected. In another method shown in FIG. 13, two flanges 21a, 21b are disposed on the peripheral ends of resin pipes 1a, 1b to be connected to form resin pipe line 1. The pipes are joined using bolts and nuts (not shown in the drawing). In another method, referring to FIG. 12, fluorocarbon resin pipes were connected by melting the outside surface of resin pipe 1a and the inside surface of resin pipe 1b, and then fitting them together.
However, the above-mentioned conventional methods have the problem of creating dead space in pipes. For example, dead space is created when the resin pipes are used to transfer pure water or ultrapure water for cooling superconductive coils and LSI manufacturing apparatuses. The method used in FIG. 12 to join the resin pipes creates dead space 1d where the two resin pipes 1a, 1b are joined. Impurities, such as water scale, accumulate in dead space 1d which is infested with bacteria. As a result, pure water or ultrapure water will also contain bacteria while in resin pipe line 1. Similarly, in the method shown in FIG. 13, it is not possible to join two resin pipes 1a, 1b completely. A small gap exists at the joint. The flanges distort a gland packing when clamped, so that the dead space created between the gland packing and the inside surface of the resin pipe has impurities such as water scale and bacteria.
In another conventional method of connecting pipes disclosed in Laid-open Japanese patent application No. (Tokkai Hei) 5-87286, two resin pipes are inserted from either side of socket-type pipe joints in which electrical resistance wires (heating wires) are embedded. Then electric current welds the pipes together. However, the usual method of manufacturing the above-mentioned socket-type pipe joints is injection molding and inserting resin-coated heating wires. This method is not only time-consuming, but also expensive due to the manufacturing pipe joints for each bore and producing a metallic mold for the injection molding.
These resin pipes are installed in ceilings or piping grooves in factories. It is not easy to work in these places. Therefore, a complicated method or apparatus can not be used to connect resin pipes.
Another method of connecting resin pipes is disclosed in Laid-open Japanese patent application No. (Tokkai Hei) 5-84829 which uses high-frequency electric power. The problem with this method is that the joints have low strength since only the end faces of resin pipes are welded together and the welded area of resin pipes is limited since the heaters are disposed at the pipe joints.